The disassembled example is mine. Sophisticated and rare. It will be renovated.

Note damage on the tip of the crescent baffle. You can just see where it has hit the recess in the contra piston.

So much intellectual effort expended in this thread!

Yes I noticed that. You could just clean it up and reassemble. Suouldn't make that much difference. I'm wondering why they went to so much effort to add that feature in the late 1960's. It doesn't look like a high performance engine, and there weren't any secrets in running a crossflow, flat topped piston diesel by then. What stops the contra from turning?

The baffle and head fit reminds me of the deep piston skirt groves in backplates like in say Rossi engines.

I love those hex headed head bolts. You can still get those in BA sizes at local "Live Steam" Model Engineering suppliers like Earnie Winter at Bathurst.

The dissassembled example is mine. Sophisticated and rare. It will be renovated.

Note damage on the tip of the crescent baffle. You can just see where it has hit the recess in the contra piston.

Doesn't surprise me with no obvious attempt made to pin the contra against rotation. Likewise I can see no reason for the holes in the topside-unless it was some misguided attempt to balance out the thermal heat capacity somewhat...

Hi, all....
First time I looked through this topic, and went all the way back to post #1... an enjoyable, if a bit long, task.

RE: Baffled piston diesel engines: I had an OS 15D in the 1950's. It had no problems of the contra-piston going out of alignment. I must have had it apart - I'm a curious type, with due skills and care for the process. Didn't recall how the contra was kept, so looked it up.

There is a site carrying published magazine test reports on engine tests, glows and diesels indexed in separate lists. Sure enough, there is a test report on the OS15D, Trick explained as a small pin in the head-clamp aligning with a short groove in the upper edge of the contra, allowing about 2mm vertical travel. The upper end mounted with 4 bolts through the clamp, secured into the crankcase casting, NOT the more usual threaded cuff screwing either onto the cylinder, or into the lower crankcase, Just sport flying CL at the time, and it ran very nicely for me with plenty of power and no fuss.

I also had an ENYA 15D-ll, and still do. The site likely still has test reports for this as D-I and D-II. I'll check. Both are "exhaust through single side of cylinder" as are almost all current 2-stroker glows.
As to the exhaustive (?_exhausting(?) discussion of piston face shapes -flat or domed or baffled.

Early in this ancient thread was implied, if not bluntly stated, reference to exhaust and bypass flow through cylinder ports. Any slight bevel helping flow change direction from straight 'down the sleeve,' has to help. How much? Hmmm.

Discussed was pressure on a conical piston crown. Much silliness written. Simply, the usable aims' down the bore w/out regard for the shapes of the piston top. The only other effects would seem to me to be internal to the actual pieces. Local pressure on the angled surface of a conical-headed piston are vectors present when combustion pressure is working. That may be 'tilted' a bit, it is only a side effect of the working pressure.

It may have more to do with more heat-absorbing surface than a flat-top piston has. Similarly, the domed or baffled surface has more metal to act as heat sink. The shift of more engines to flat piston faces seemed more likely to reduce production steps, thus costs, as performance was NOT decisively different.

Another thread in this area discusses home-brewed diesel fuels. Lubrication in particular. Several claimed poorer than expected RPM after head conversion of available glow engines. Mention was made that modern glow engines deserve 'modern' fuels. Not mentioned was the difference between ferrous and non-ferrous engines cyl / piston components. One stated that the 'classic' 1:1:1 blend was unsatisfactory. Another mentioned that 23% oil was adequate.

In the 'classic' blend, one-third is castor oil. Most older engines, and still - I believe - most designed-as diesels have iron & steel type top ends. Most, if not ALL newer glow engines are non-ferrous. They are ABC, ABN, AAC, etc.

Non-ferrous engines have different heat conditions best suited to use, power and life-span. Designed and manufactured to fit ideally in a specific heat range. Expansion to ideal fits is essential. Irons and steels expand less, so physical clearance fits are less likely depend on relative expansion of aluminum alloy and brass metals. We've all heard advice that running an "A?C" engine too slowly and oily can damage its ultimate power, ease of use and durability, because they were broken-in too cool and too oily to reach the heat zone they were designed for. Parts that don't expand enough can wear more quickly.

That is the intention. Bottom end is good. German races are nice and smooth. The pulverisation of the baffle tip caused particles of cast iron to lap the cylinder. It needs a rebore. I have asked Mr Fletcher (whom I trust implicitly) to do that and to look at replicating the curved baffle. He supplied the photographs shown above. Anodised parts will be cleaned back to bare and re-anodised.

As grandad told me when I was a pup, you don't spoil the ship for a ha'porth o' tar.

Hi, all....
First time I looked through this topic, and went all the way back to post #1... an enjoyable, if a bit long, task.

Discussed was pressure on a conical piston crown. Much silliness written. Simply, the usable aims' down the bore w/out regard for the shapes of the piston top. The only other effects would seem to me to be internal to the actual pieces. Local pressure on the angled surface of a conical-headed piston are vectors present when combustion pressure is working. That may be 'tilted' a bit, it is only a side effect of the working pressure.

\LOU)

Hi Lou,
long time since I have commented in this thread but since many of the responders are still around I don't feel so bad.
Question, define "silliness."

Pascals law (which seems to be the crutch that holds this one up) only applies well if the fluid is incompressable and static, and with fluid in a model engine that compresses and expands in cycles measures in the thousands per minute, applying this thinking is flawed.
If flow is contoured or flowing in any way and especially if its cyclic, then pressures within the bounded area will not be the same everywhere at any time.
Where there is a baffle, pressure differences can and will happen, where gas is forced to expand and move around a shape, such as a conical moving piston you will suffer a loss as a result of an action.
A conical crown will angle an expanding charge away from the direction of travel into the contact area between the cylinder wall and the piston creating a loss, far better to have it only directed parrallel to the direction of travel.

Maybe the effect is only slight in practice but I believe that any convex shaping of the combustion chamber, and this includes the crown, is adverse to any HCCI engine that endevours to ignite the charge all at once. In contrast concave shaping seems to be beneficial to this process due to the reasons given above.

Hi Lou,
long time since I have commented in this thread but since many of the responders are still around I don't feel so bad.
Question, define "silliness."

Pascals law (which seems to be the crutch that holds this one up) only applies well if the fluid is incompressable and static, and with fluid in a model engine that compresses and expands in cycles measures in the thousands per minute, applying this thinking is flawed.
If flow is contured or flowing in any way and especially if its cyclic, then pressures within the bounded area will not be the same everywhere at any time.
Where there is a baffle, pressure differences can and will happen, where gas is forced to expand and move around a shape, such as a conical moving piston you will suffer a loss as a result of an action.
A conical crown will angle an expanding charge away from the direction of travel into the contact area between the cylinder wall and the piston creating a loss, far better to have it only directed parrallel to the direction of travel.

Maybe the effect is only slight in practice but I believe that any convex shaping of the combustion chamber, and this includes the crown, is adverse to any HCCI engine that endevours to ignite the charge all at once. In contrast concave shaping seems to be beneficial to this process due to the reasons given above.

Cheers.

Regardless of the various laws that may or may not apply-there is documented evidence that the combustion process and gas flow in 360 degree ported diesels is not symmetric-clear evidence from the pattern of carbon deposits on the piston crown-and from a different line of experimentation-the use in some diesels of a squish band on the contrapiston delivers smoother running. I've also witnessed first hand the effect on an ED Comp Spl of grinding a shallow hemisphere into the contra piston combustion face-the running improved significantly in both smoothness and reduction of vibration. How such effects might play out in the case of a baffled piston is entirely another matter-and I doubt anyone has the inclination to investigate it in detail-with all the variables possible-four different baffle types for a start-5 if you include step baffles...

Hi Chris,
is it the inclusion of the squish band that gives smoother running or the inclusion of a concave shape?

Always thought that the terminolgy of a "squish band" is misapplied in an HCCI engine and its more that the hollowing out of the head leaves by default an area untouched.

Thanks.

I don't know-it would seem that there are three basic geometries to be explored-a squish band on a flat contra face, a squish band on a conical contra face and a squish band on a hemispherical contraface (though I use the term 'hemispherical' with some hesitation-only because it is in generic use to described a rounded concave curved combustion chamber shape cut with a ball ended milling cutter)-it would be equally valid and more accurate to describe a 'squish band with a dished contraface'...)-and does-by default-the machining of any concave face profile on a contrapiston of less than full bore diameter-create a squish band? IIRC the squish band concept was developed-for model engine use-in the early 60s-by a few E European fliers-and took much much longer to come into general use in glow engines.

Personally-I am inclined to the view that it is the hollowing out that is important-but you instantly realise that to investigate this in systematic detail would require machining a variety of contrapistons varying in both width and depth and shape of the depression-and given the criticallity of fits-an enormous amount of work for what might in the end be inconclusive results. Eventually you would reach a point where you could not generate enough compression to fire the charge....

I can rationalise the effect in terms of promoting a more staged ignition of the fresh charge rather than a random detonation as would be expected with a dead flat contra-and something of the order of an actual burn rather than a detonation-and the failure of many dieselized glow engines-crankshafts and rods/gudgeons when operated as diesels is proof enough that the diesel combustion process is a lot more brutal than the burning of methanol under glow ignition in the same engine

One way would be perhaps to experiment with a typical converted glow engine-and the DDD type of head-using a sub bore diameter contra and O-ring seal. This would allow the creation of different diameters and shapes of sub bore size moveable contras and shed some light-but again I suspect the results would only be valid for the modern schneurle converted glow engine type of diesel. By mixing metric and imperial O-ring szes it should be possible to cover a reasonable variation in moveable contra size up to full bore diameter. And given that things like the Gadjet once existed-and presumably worked...up to a point-it seems that the minimum workable variable contra diameter size could be quite small.....

I don't know-it would seem that there are three basic geometries to be explored-a squish band on a flat contra face, a squish band on a conical contra face and a squish band on a hemispherical contraface (though I use the term 'hemispherical' with some hesitation-only because it is in generic use to described a rounded concave curved combustion chamber shape cut with a ball ended milling cutter)-it would be equally valid and more accurate to describe a 'squish band with a dished contraface'...)-and does-by default-the machining of any concave face profile on a contrapiston of less than full bore diameter-create a squish band? IIRC the squish band concept was developed-for model engine use-in the early 60s-by a few E European fliers-and took much much longer to come into general use in glow engines.

Personally-I am inclined to the view that it is the hollowing out that is important-but you instantly realise that to investigate this in systematic detail would require machining a variety of contrapistons varying in both width and depth and shape of the depression-and given the criticallity of fits-an enormous amount of work for what might in the end be inconclusive results. Eventually you would reach a point where you could not generate enough compression to fire the charge....

I can rationalise the effect in terms of promoting a more staged ignition of the fresh charge rather than a random detonation as would be expected with a dead flat contra-and something of the order of an actual burn rather than a detonation-and the failure of many dieselized glow engines-crankshafts and rods/gudgeons when operated as diesels is proof enough that the diesel combustion process is a lot more brutal than the burning of methanol under glow ignition in the same engine

One way would be perhaps to experiment with a typical converted glow engine-and the DDD type of head-using a sub bore diameter contra and O-ring seal. This would allow the creation of different diameters and shapes of sub bore size moveable contras and shed some light-but again I suspect the results would only be valid for the modern schneurle converted glow engine type of diesel. By mixing metric and imperial O-ring szes it should be possible to cover a reasonable variation in moveable contra size up to full bore diameter. And given that things like the Gadjet once existed-and presumably worked...up to a point-it seems that the minimum workable variable contra diameter size could be quite small.....

ChrisM
'ffkiwi'

Modern racing diesels with push pull contra heads are mostly dead flat. Didn't we discuss the earlier in the thread? Or was that another?

I think that it was on the Parra web site that claimed that a dead flat or a 0.3mm combustion chamber step to the contra gave the best runs.

The late and great Pe Rievers was also on record as stating the same.

A point of history here, wasnt it Ken Bedford of ETA fame that first used a "squish band" type of head successfully in racing?
(If you can confirm the answer to this then you are showing your age like me! Looking at you Ray and Chris.)

l"Your engine has been factory set with a clearance of approximately 0,28 - 0,30 millimetres between the cylinder head and piston at the top of its stroke. This is optimum for operation at sea level with the recommended propeller and fuel mixture given for running-in. Adjustment of the contra-piston via the compression adjusting screw will give a wide range of operating speeds. If you need more compression than is available, remove a shim, back off the contra-piston a little (to allow for fine tuning) and reassemble. The engine will run satisfactorily with the contra-piston recessed back in the head. However, the best combustion chamber shape is when the contra-piston is flush with the head, or recessed slightly (about 0,3 millimetres), which allows for some fine-tuning. Once you have settled on fuel composition and propeller andestablished the in-flight compression setting, then remove the head and inspect the contra-piston position. If it is significantly recessed, add a shim and adjust the contra-piston down to restore the desired compression ratio. It may require several such adjustments to arrive at the ideal position."

I think that it was on the Parra web site that claimed that a dead flat or a 0.3mm combustion chamber step to the contra gave the best runs.

The late and great Pe Rievers was also on record as stating the same.

A point of history here, wasnt it Ken Bedford of ETA fame that first used a "squish band" type of head successfully in racing?
(If you can confirm the answer to this then you are showing your age like me! Looking at you Ray and Chris.)

l"Your engine has been factory set with a clearance of approximately 0,28 - 0,30 millimetres between the cylinder head and piston at the top of its stroke. This is optimum for operation at sea level with the recommended propeller and fuel mixture given for running-in. Adjustment of the contra-piston via the compression adjusting screw will give a wide range of operating speeds. If you need more compression than is available, remove a shim, back off the contra-piston a little (to allow for fine tuning) and reassemble. The engine will run satisfactorily with the contra-piston recessed back in the head. However, the best combustion chamber shape is when the contra-piston is flush with the head, or recessed slightly (about 0,3 millimetres), which allows for some fine-tuning. Once you have settled on fuel composition and propeller andestablished the in-flight compression setting, then remove the head and inspect the contra-piston position. If it is significantly recessed, add a shim and adjust the contra-piston down to restore the desired compression ratio. It may require several such adjustments to arrive at the ideal position."

Quote:

... or recessed slightly (about 0,3 millimetres), which allows for some fine-tuning.

I think it's saying that the slightly recessed contra is just a starting position. Dead flat is aspirational which may not be mechanically possible without fine shimming. That is also confirmed by local experts :-)

I don't know about ETA diesels. I have one I inherited, but have never looked at it closely.

I've re-read the thing again, and at least glanced at the images to refresh...

About the combustion pressure on cylinder interior surfaces - of course it is ideally equal everywhere and perpendicular to the surfaces it meets. If the piston does not move, by definition, no work is done. I use the word ideally because the events happen in such brief time that we may need to consider flame - or is it more the combustion - progression.

In a spark or hot wire catalyzed 'burn' we have a fair idea of when combustion starts and where. There have been studies of flame progression done by the US Society of Automotive Engineers many years ago. Glass cylinder head or some such. They included studies of flame front progression. At the time, these were for modest RPM, low compression, gasoline (petrol) burning engines with valves, and probably the spark plug, outside the cylinder diameter in a side chamber.

They recorded flame propagation speed and characteristics. Clearly, ignition began at the 'trigger' point, the spark plug gap. In a 'glow plug' engine. I'd presume much the same applies. The catalyst wire is hot enough to act as trigger location, Gasoline and kerosene (paraffin) flame progression were both slower than alcohols. I seem to recall they tested what we refer to as wood alcohol - methanol. Burning ideal proportions of alcohol and oxygen, the equipment used then could not measure the propagation speed. Speeds for petrol and paraffin were measured. (Our fuels are quite unlike the tested fuels - we blend in oil and do not run to maximum potential.)

Consider: The time available to initiate the most useful portion of the 'burn' is extremely brief. E.g., at 18,000 RPM, that's 300 full rotations per second 108,000 degrees per second. If the most effective portion of the 'burn' lasts from 15° before TDC to about 25° after TDC, about 40°, the actual time involved would be 0.00037 seconds. Even in the small distances in our combustion chambers, it may still take significant time for the pressure to equalize to all exposed surfaces...

Initiation, propagation and early decline of pressure all occur in that duration. Strobe photography can capture the initiation, progression and waning of the event. (I assume not during one event, but at sequential iterations delayed by defined degrees of shaft rotation. Not sure, but I doubt that other adequate methods existed when SAE did these studies in the 1920's and 1930's. We have advanced, since, but the basics of the combustion progress remain.)

40° may be a generous guess at the shaft degrees involved. The pressure falls as combustion completes and as the piston moves down increasing the trapped volume from its optimums during the burn. Piston motion is, of course, the work combustion produces..

Do we actually know if paraffin burns during our model diesel combustion? Specifically, does the process include actual flame? Petrol and methanol we must presume to burn with a definite flame. In dim light, the hot orange light in the exhaust is undeniable. Does that also apply to our diesels?

Re: the baffled piston OS 15D - the projection was not a reciprocating piece, it was an alignment peg that slid into the recess in the mating part only during reassembly. That emerged clearly over several subsequent posts.

I think that it was on the Parra web site that claimed that a dead flat or a 0.5mm combustion chamber step to the contra gave the best runs.

The late and great Pe Rievers was also on record as stating the same.

A point of history here, wasnt it Ken Bedford of ETA fame that first used a "squish band" type of head successfully in racing?
(If you can confirm the answer to this then you are showing your age like me! Looking at you Ray and Chris.)

But as I noted earlier-how systematic was the Parra testing?-I already outlined some of the variables-and you would need to test these independently-ie shapes separately from volume separately from diameter........and this thread was largely about baffled piston diesels and the merits or lack thereof-plus the same regarding conical head piston and contras. I have the same issue when someone asks about the best head setup for XYZ motor for FF. Ok-on what prop, on what fuel, at what T&H, what plug, what head clearance-to get a truly valid answer you'd have to spend days and days testing-and that presumes you had an adequate supply of head shims and a range of plug types at your disposal-rather than the usual situation of what you happened to have to hand.

I operate on the basis-when it comes to engines-that no one does any more work than they have to, when it comes to manufacture-unlike people who build their own-either for personal satisfaction or because they get total control over both the design and execution. If manufacturers were using conical pistons and matching contras in the 50s-I think it was because there was a good chance that it was because someone had found that that gave better results than a flat top setup-for the porting used at the time!'....exactly the same argument applies to SPI-which was equally fashionable around the same period-in case cases-eg Elfin...being taken to extreme lengths. Did anyone actually sit down and make a whole range of conical pistons and matching contras varying only the cone angle and systematically test? Probably not-but the chances are fairly high that they guessimated an initial attempt and tried a couple of different cone angles.... Then you have the situation where yes-there IS a demonstrable effect-but its small-and in the scheme of things it easier from a production viewpoint to make them flat and live with the marginal loss of performance-and in any case the engine to engine variation across production was probably more than the initial performance variation anyway.....and ease of production and cost minimization wins the day every time....this would certainly be sufficient to account for cases like the Allbon Dart and Javelin-whereas the Germans in particular were very much wedded to the conical piston/contra approach-as evidenced by ALL my Waifun and Webra diesels having conical pistons-while Elfin and AM also stuck with the concept.

PAW I believe took up the squish approach in the late 60s ....I haven't checked their adverts in depth-but the Oct 67 AM I have out on the table reads "now with squish head-for silenced running with minimum power loss"-in reference to their 'new' 2.49 Mk4 model-it was not specified whether the squish head migrated into the 149 and 19 models at a later date....

I've re-read the thing again, and at least glanced at the images to refresh...

About the combustion pressure on cylinder interior surfaces - of course it is ideally equal everywhere and perpendicular to the surfaces it meets. If the piston does not move, by definition, no work is done. I use the word ideally because the events happen in such brief time that we may need to consider flame - or is it more the combustion - progression.

In a spark or hot wire catalyzed 'burn' we have a fair idea of when combustion starts and where. There have been studies of flame progression done by the US Society of Automotive Engineers many years ago. Glass cylinder head or some such. They included studies of flame front progression. At the time, these were for modest RPM, low compression, gasoline (petrol) burning engines with valves, and probably the spark plug, outside the cylinder diameter in a side chamber.

They recorded flame propagation speed and characteristics. Clearly, ignition began at the 'trigger' point, the spark plug gap. In a 'glow plug' engine. I'd presume much the same applies. The catalyst wire is hot enough to act as trigger location, Gasoline and kerosene (paraffin) flame progression were both slower than alcohols. I seem to recall they tested what we refer to as wood alcohol - methanol. Burning ideal proportions of alcohol and oxygen, the equipment used then could not measure the propagation speed. Speeds for petrol and paraffin were measured. (Our fuels are quite unlike the tested fuels - we blend in oil and do not run to maximum potential.)

Consider: The time available to initiate the most useful portion of the 'burn' is extremely brief. E.g., at 18,000 RPM, that's 300 full rotations per second 108,000 degrees per second. If the most effective portion of the 'burn' lasts from 15° before TDC to about 25° after TDC, about 40°, the actual time involved would be 0.00037 seconds. Even in the small distances in our combustion chambers, it may still take significant time for the pressure to equalize to all exposed surfaces...

Initiation, propagation and early decline of pressure all occur in that duration. Strobe photography can capture the initiation, progression and waning of the event. (I assume not during one event, but at sequential iterations delayed by defined degrees of shaft rotation. Not sure, but I doubt that other adequate methods existed when SAE did these studies in the 1920's and 1930's. We have advanced, since, but the basics of the combustion progress remain.)

40° may be a generous guess at the shaft degrees involved. The pressure falls as combustion completes and as the piston moves down increasing the trapped volume from its optimums during the burn. Piston motion is, of course, the work combustion produces..

Do we actually know if paraffin burns during our model diesel combustion? Specifically, does the process include actual flame? Petrol and methanol we must presume to burn with a definite flame. In din light, the hot orange light in the exhaust is undeniable. Does that also apply to our diesels?

Re: the baffled piston OS 15D - the projection was not a reciprocating piece, it was an alignment peg that slid into the recess in the mating part only during reassembly. That emerged clearly over several subsequent posts.

Lou-all good points-and the simple answer is: I don't think anyone has actually looked. I do know that one of my Frog 249s from time to time emits glowing sparks from the exhaust when running-but whether anything can be inferred from this I doubt. I suppose-at least in theory you could make a glass contrapiston and monitor combustion through a micro camera inserted in a hollow comp screw. There are Youtube videos of glow engines running with transparent components:

One avenue that might shed some light is if a pressure transducer/transducers was/were fitted into the head and cylinder walls in a radial pattern-with a very high duty cycle so we could get an indication of pressure peaks during the diesel combustion process-this might shed some light on whether the initiation of combustion is a random event or occurs in a fixed location-and the type of burn....

ChrisM
'ffkiwi'

PS If you look at the first two videos links posted-it is quite surprising how 'messy' the charge mixture is-making me wonder just how well our engines atomise the fuel mixture during carburetion...it would seem that a LOT more work could be done in improving gas flows internally...

Actually, the idea of seeing the hot glow was a random thought. In the early '70s I was in Nebraska. We had a fine, paved circle in a truck overflow parking lot for a large motel. Railroads and local conditions - a major amusement park and little residential housing - allowed us weekday evenings. Flew late into dusk on occasion. Particularly as we entered the earlier darkness approaching Fall,

Inverted Fox 35. No muffler. BRIGHT orange in the exhaust opening. Could have been the plug filament lighting the combustion volume. Could have been flame. Impressive whatever it was.

Just a thought. Thanks for the replies checking the possibility! I expect most of the flaming burn - if any - would go to completion before the exhaust port uncovers on our diesels.
\LOU

Same here, as a kid flying glows on the oval behind my house right into dusk on a summers evening. Bright exhaust glow. Of course a Mills 1.3 is right down one end of the spectrum as diesels go. I'll ask the question at the next club get together. We have several hundreds of years of diesel experience there on a good day :-)